DUNDALK, Ireland >> Gerard McEvoy’s daily commute across the border between his home in the United Kingdom and his workplace in Ireland takes just 10 minutes, and the only hint that he is in a different country is when the road signs change from miles to kilometers.

But that all-but-invisible border between Northern Ireland and Ireland may end up a victim of Britain’s decision in June to leave the European Union. With Ireland still a member of the bloc, a new arrangement for the flow of people and goods will have to be negotiated. And the border is just one challenge in what is likely to be a much broader redefinition of the relationship between Britain and Ireland, another example of the sprawling and often unintended consequences of Britain’s choice to split from Europe.

Partly as a result of their shared membership in the European Union, London and Dublin have largely put aside historical animosities and developed a tightly woven relationship. Now Britain’s exit from the bloc holds the risk of introducing new friction, from the economy to the management of Northern Ireland’s sectarian tensions to the familial, social and cultural ties that bind the inhabitants of the islands.

“The border has been out of sight, out of mind — there’s been no trouble,” McEvoy said, standing in his department store in Dundalk and recalling how he grew up during the Troubles.

Those were years of strife and violence in Northern Ireland between some Protestants who wanted to remain part of Britain and some Roman Catholics who favored unification with Ireland. But McEvoy added, “With Europe, you could be as Irish as you wanted to be, you could be British — whatever you wanted to be.” His comment reflected the broad support for membership in the European Union in Northern Ireland, which voted 56 percent to 44 percent to stay in the bloc.

Four decades of European integration have helped Ireland not only escape the shadow of Britain, but also improve relations with London and work with the British for peace in Northern Ireland. Now the question is whether Britain’s departure from the bloc will drive a wedge between them.

The return of a hard border could affect the fragile peace process, with Sinn Fein, the dominant party among Catholics in Northern Ireland, using the Brexit vote as reason to call for a referendum on uniting Northern Ireland and Ireland. At the same time, Irish politicians also worry about the economic stability of Northern Ireland, which depends heavily on subsidies from London and the European Union. They wonder whether British taxpayers will pick up the tab for cuts in European funding.

“I think that it’s an enormous moment and potentially a catastrophic moment in terms of Ireland’s narrow interests as well as those of the wider European Union,” said Eunan O’Halpin, a professor of contemporary Irish history at Trinity College Dublin. “Our neighbors have burned the house down, and once the edifice collapses, we have to see how we can fix our walls.”

Ireland’s foreign minister, Charles Flanagan, acknowledged the gravity of the change. Ireland, he said, has become “a totally different place” from the country that joined the forerunner of the European Union on the same day as Britain in 1973.

Yet even an economically transformed Ireland, he said, cannot escape the ramifications of a referendum by a large neighbor with whom it shares centuries of troubled history.

“There are potential negative impacts across every government department, from energy to agriculture, the environment,” Flanagan said at his office in Dublin. “Our job in the negotiations will be to mitigate these losses and minimize the damage.”

Pat Cox, a former president of the European Parliament, said the European dimension played a crucial role in strengthening relations between Dublin and London. Membership made Ireland important in its own right and allowed it to differentiate itself from Britain, and to pick and choose policies best suiting its interests.

Like the British, the Irish stayed out of the passport-free Schengen travel zone, but unlike Britain, Ireland joined the European single currency. A shared, free-market economic perspective, however, made the two nations allies in Brussels.

“We got to know each other very well over 40 years by showing up at the same tables in Brussels and elsewhere, not on the basis of equal size but on the basis of equality of status of member states, which for a small state matters,” Cox said.

“This radically changed the quality of dialogue between Irish and British leaders over time,” he added, while fostering the “mutual respect and understanding” that made the Northern Ireland peace process possible.

Yet its closeness to Britain leaves Ireland exposed to the consequences, particularly for trade, of Britain’s withdrawal from the European Union. “In a worst-case scenario with the U.K. outside the EU, the impact could be 20 percent or more,” Ireland’s Economic and Social Research Institute concluded last year. It added that a Brexit would be a huge blow since “more than 15 percent of Irish exports are destined for the U.K.”

If Britain leaves the European Union’s single market, as seems likely to happen, trading with the British would most likely involve new customs requirements, and possibly tariffs.

For many in Britain, a prime motivation for the vote was to “take back control” of its borders. If Britain does leave the union, it will be difficult to avoid immigration checks at the Irish border or between Northern Ireland and mainland Britain, though the latter would be controversial in Belfast.

Flanagan described a heavily fortified border between Ireland and Northern Ireland, with immigration controls, as “unthinkable,” but said he could not rule out some customs checks. “If there are to be some customs points, modern technology could be used to the full in terms of the identification and checking of goods,” he added.

© 2016 The New York Times Company